Modulating FRET in Organic–Inorganic Nanohybrids for Light Harvesting Applications

The energy transfer efficiencies of organic–inorganic nanohybrids comprised of two structurally similar squaraine dyes and CdSe nanoparticles were studied in detail and compared. Carbazole based unsymmetrical squaraine dyes (CTSQ-1 and CTSQ-2) having modified absorption characteristics were considered for modulating the effect of the overlap integral on energy transfer rate with the designed QDs. CTSQ-2 with ∼1.75 times higher molar extinction coefficient and 35 nm red-shift in absorption resulted in an ∼2.4 times faster energy transfer rate with QD. The calculated energy transfer rates (<i>k</i>_T = 1.35 × 10⁸ s^–1 and 3.26 × 10⁸ s^–1 respectively for QD:CTSQ-1 and QD:CTSQ-2 nanohybrids) are at least one order of magnitude higher than both radiative (<i>k</i>_r = 5.97 × 10⁶ s^–1) and nonradiative decay rate constants (<i>k</i>_nr = 1.89 × 10⁷ s^–1) of QDs yielding very high FRET efficiency. The Stern–Volmer analysis of the quenching data indicated mainly static interaction of dyes with the QDs thus suggesting formation of organic–inorganic nanohybrids. When incorporated in dye-sensitized solar cells, the nanohybrids with 93% FRET efficiency, exhibited an overall 43% improvement in the photovoltaic performance. Among the two architectures employed for device fabrication the one with the smallest donor–acceptor distance delivered the best performance. Due to increased contribution from QDs, the IPCE spectra clearly indicate panchromatic response from the visible to NIR region. Thus, photovoltaic performance of NIR absorbing dyes were successfully improved by constructing panchromatic organic–inorganic nanohybrid materials

Probing Recombination Mechanism and Realization of Marcus Normal Region Behavior in DSSCs Employing Cobalt Electrolytes and Triphenylamine Dyes

Cobalt based, outer-sphere, one-electron redox shuttles represents an exciting class of alternative electrolyte to be used in dye-sensitized solar cells. The flexibility of redox potential tuning by varying the substituents on peripheral organic ligands renders them the advantage of achieving higher photovoltage. However, higher recombination experienced in these systems by employing diffusion-limited cobalt species serves as a bottleneck which significantly limits attaining higher performance. The focus of the present contribution is to systematically investigate in detail the effect of structural variations and steric hindrance of organic triphenylamine dyes (TPAA4 and TPAA5) which differs in the number and nature of binding groups and peripheral hole accepting units on the recombination reactions and mass transport variations employing two different cobalt electrolytes, [Co₃]^3+/2+ and [Co­(phen)₃]^3+/2+, having variable driving force for recombination. The detailed photovoltaic analysis provides us the information that modification of the architecture of organic dyes plays a decisive role in determining the performance, in particular, employing alternate one-electron outer-sphere redox systems. From our analysis, for both the dyes the charge recombination with the oxidized cobalt species was found to happen in the Marcus normal region which is attributed to the shift in conduction band (CB) that influenced the driving force for recombination. The current observation was quite exciting since the redox systems employed in the present study were previously documented to exhibit Marcus inverted recombination behavior. The impact of structural variations of dyes, change in conduction band, effect of nature of electrolyte species, and its interaction with the semiconductor on the recombination reactions was explored in detail using a range of small and large perturbation techniques